1. Field of Invention
The invention relates to an image forming apparatus which supplies toner to an electrostatic latent image formed on a sensitive body of a developing unit to form a toner image and transfers the toner image to a paper sheet and then heats it via a fixing unit to thermally fix the toner image and to form an image. In particular, the invention relates to an image forming apparatus which can prevent the toner, paper powder or other contaminants sticking to an image non-forming surface in the developing unit from sticking to the inner wall of the cover of the developing unit and the inner wall of a paper introduction port of the fixing unit and can smoothly transfer the paper sheet from the developing unit to the fixing unit, whereby a paper jam can be prevented with reliability.
2. Description of Related Art
Various kinds of image forming apparatuses, such as laser printers or the like, using an electrophotography method, have been proposed. The specification and drawings of a Japanese Published Patent Document No. 9-319201, for example, describes an image forming apparatus categorized broadly into a paper carrying unit, a laser scanning unit, a toner supply unit, a developing unit, a fixing unit and the like. In such an image forming apparatus, an electrostatic latent image is formed on the surface of a photosensitive drum in the developing unit, according to the image data, by the laser scanning unit and toner is supplied to the photosensitive drum by the toner supply unit to form a toner image, according to the electrostatic latent image, on the surface of the photosensitive drum, and the toner image is transferred to the paper sheet carried into the developing unit from the paper carrying unit by a transfer roller and the photosensitive drum. The paper sheet is then heated by the fixing unit to fix the toner image.
When the image is formed in this way, if the paper sheet carrying speed of the fixing unit side is set larger than the paper sheet carrying speed of the developing unit side, an image forming position on the paper sheet might be shifted. Therefore, in general, the paper carrying speed of the developing unit side is set larger than the paper sheet carrying speed of the fixing unit side. Accordingly, there is a tendency for the paper sheet to which the toner image is transferred to be carried to the fixing unit side while it makes contact with the inner, upper wall of the cover of the developing unit and that, after it makes contact with the top side of a paper sheet introduction port formed in the cover of the fixing unit, it is carried into the fixing unit.
In a conventional image forming apparatus, however, the toner, paper powder and other contaminants are dispersed in the developing unit. Therefore, while the paper sheet is carried in the developing unit, the toner and, the paper powder result in sticking to the image non-forming surface of the paper sheet, i.e., top surface of the paper sheet as transported. Moreover, since the paper sheet carrying speed of the developing unit is set larger than the paper sheet carrying speed of the fixing unit, the paper sheet is carried toward the fixing unit side while it comes into contact with the inner wall of the cover of the developing unit and, because it comes into contact with the top side of the paper sheet introduction port formed in the cover of the fixing unit, the toner and the paper powder sticking to the paper sheet stick to the inner upper wall of the cover of the developing unit and the top end portion of the paper sheet introduction port of the cover of the fixing unit.
The toner and the paper powder sticking in this way prevent the paper sheet from being smoothly carried from the developing unit to the fixing unit and, in particular, if the toner and the paper powder stick to the paper sheet introduction port of the fixing unit, there arises a problem that a paper sheet jam might occur at the paper sheet introduction port.
It is an object of the invention to overcome the above-described problems, and to prevent toner, paper powder and other contaminants, sticking to an image non-forming surface of a paper sheet in a developing unit, from sticking to the inner wall of the cover of a developing unit and the inner wall of a paper sheet introduction port. Another object of the invention is to smoothly carry the paper sheet from the developing unit to a fixing unit and to reliably prevent a paper sheet jam.
In order to accomplish the above-described objects, in an image forming apparatus according to one aspect of the invention, there is provided an image forming apparatus comprising developing means which supplies toner according to an electrostatic latent image formed on a sensitive body and forms a toner image, transfer means which is disposed opposite to the above-described sensitive body and transfers the toner image formed on the sensitive body to a paper sheet carried by a paper sheet carrying unit, and fixing means which heats and fixes the toner image transferred to the paper sheet, and further comprises a first guide member which is disposed in the downstream side of the above-described transfer means in the direction that carries the paper sheet along the carrying path of the paper sheet and which comes into contact with the lead edge of the paper sheet on which the above-described toner image is formed and directs the lead edge of the paper sheet toward the above-described fixing means, and a second guide member which is disposed in the upstream side of the above-described fixing means in the direction that carries the paper sheet along the carrying path of the paper sheet and which comes into contact with the lead edge of the paper sheet on which the above-described toner image is formed and introduces the lead edge of the paper sheet into the above-described fixing means.
In the above image forming apparatus, the paper sheet to which the toner image is transferred by the sensitive body in the developing means is carried toward the fixing means along the paper sheet carrying path while the surface of the lead edge thereof comes into contact with the first guide member. The paper sheet discharged by the developing means is introduced into the fixing means while the surface of the lead edge thereof comes into contact with the second guide member.
At this time, the toner and the paper powder dispersed in the developing means stick to the image non-forming surface of the paper sheet via the top surface of the first guide member, but the amount of such contaminants is extremely small in comparison with the amount of the toner and the paper powder when the toner and the paper powder sticking to the whole bottom surface of the cover of the developing means might stick to the paper sheet and hence there is no problem. Therefore, the toner and the paper powder scarcely stick to the apex surfaces of the first guide member and the second guide member. Accordingly, it is possible to prevent the toner and the paper powder from sticking to the inner wall of the cover of the developing means and the inner wall of the paper sheet introduction port of the fixing means and to smoothly carry the paper sheet from the developing means to the fixing means via the first guide member and the second guide member, which reliably prevents a paper sheet jam.
Moreover, the image forming apparatus is preferably characterized in that the above-described first guide member and the above-described second guide member in the above image forming apparatus are mounted plurally in the direction which intersects the carrying path of the above-described paper sheet, and that the above-described first guide member and the above-described second guide member are staggered from each other. Since a plurality of first guide members and a plurality of second guide members are staggered from each other in the image forming apparatus of the invention, they are not aligned. Therefore, even if the toner and the paper powder sticking to the apex surfaces of the first guide members stick to the paper sheet, it is possible to reliably prevent the toner and the paper powder from sticking to the second guide members. Accordingly, the toner and the paper powder never fixedly stick to the inner wall of the paper sheet introduction port of the fixing means.
Further, in the above image forming apparatus, the first carrying speed of the paper sheet expelled by the above-described developing means is set equal to or larger than the second carrying speed of the paper sheet expelled by the above-described fixing means and that, when the above-described paper sheet is carried, the lead edge of the paper sheet comes into contact with the first guide member and then comes into contact with the second guide member, and is guided into the above-described fixing means along the slant of the second guide member. In the image forming apparatus of the invention, the lead edge of the paper sheet comes into contact with the first guide member and then comes into contact with the second guide member and is then guided into the fixing means along the slant of the second guide member, as described above, based on a relationship between the first carrying speed of the developing means and the second carrying speed of the fixing means. In this respect, as described above, the paper sheet is smoothly carried from the developing means to the fixing means by the first guide member and the second guide member without sticking the toner and the paper powder to the inner wall of the cover of the developing means and the inner wall of the paper sheet introduction port of the fixing means.
Preferably, in the above image forming apparatus, a discharging film is disposed between the above-described first guide members. In the above image forming apparatus, the paper sheet is effectively discharged at the developing means side by the discharging film and hence it is possible to prevent the toner and the paper powder sticking to the paper sheet. Furthermore, the image forming apparatus is preferably characterized in that the above-described first guide member is composed of a first rib formed on the paper sheet discharge portion of the cover of the above-described developing means and that the above-described second guide member is composed of a second rib formed on the paper introduction portion of the cover of the above-described fixing means. In addition, in the image forming apparatus, the above-described transfer means is disposed above the above-described paper sheet carrying path and that the above-described first rib protrudes downward from the cover of said developing means and that the above-described second rib protrudes downward from the cover of the above-described fixing means.
In the image forming apparatus, the first rib as the first guide member can be integrally formed with the cover of the above-described developing means in a state where it protrudes downward and the second rib as the second guide member can be integrally formed with the cover of the above-described fixing means in a state where it protrudes downward.